In the past, as a method of transferring a toner image on an electrophotographic photoreceptor (hereinafter, also referred to simply as a photoreceptor) onto a recording material for a final image, there has been known a method of direct transfer of a toner image formed on an electrophotographic photoreceptor onto a recording material. On the other hand, there is known an image forming system, the system employing an intermediate transfer member, in which a transfer process of transferring a toner image from an electrophotographic photoreceptor to a recording material incorporates another transfer process, wherein the toner image is primarily transferred from the electrophotographic photoreceptor to the intermediate transfer member, then the primary transfer image in the intermediate transfer member is secondarily transferred to the recording member, thereby the image forming system obtaining a final image. An intermediate transfer system as described above is mostly employed as a superimposing transfer system that superimposes toner images of respective colors in a so-called full color image forming apparatus, wherein the superimposing transfer system reproduces an original image, the original image having been color-separated, with use of a subtractive mixture of toners of black, cyan, magenta, yellow, etc.
However, in the case of a superimposing transfer system fitted with this intermediate transfer member, image failure caused by transfer failure of a toner image tends to be generated since transferring twice such as the primary transfer and the secondary transfer is conducted, and four color toners are also superimposed on the intermediate transfer member.
It is commonly known that transfer efficiency can be improved by treating the toner surface with additives such as silica and the like against transfer failure of toner. However, there is a problem such that sufficient transfer efficiency can not be obtained since silica is removed from the toner surface, and embedded inside the toner because of stress influenced from a stirring member of toner in a developing device, stress influenced from a regulation blade to form a toner layer on a developing roller, stress influenced between a photoreceptor and a developing roller, and so forth.
In order to solve such the problem, it is disclosed that detachability of toner images is improved by coating silicon oxide, aluminum oxide or the like onto the intermediate transfer member surface, whereby transfer efficiency to a recording paper sheet or such is tried to be improved (refer to Patent Documents 1 and 2, for example).
Patent Document 1: Japanese Patent O.P.I. Publication No. 9-212004
Patent Document 2: Japanese Patent O.P.I. Publication No. 2001-347593